The transfer of bulk energy through alternating voltages and currents is inherently done at some nominal frequency, typically at 50 or 60 Hz. Historically, small variations in the nominal line frequency were of little concern to electromechanical watthour metering. Electromechanical meters were limited to basic metrics such as watthours or VARhours using phase shifting transformers, and the accuracy of the results were not generally dependent on frequency.
The recent deregulation of the utility industry has created a market for products that facilitate the efficient distribution and monitoring or electrical power. In the past, utilities have built an infrastructure that did not provide adequate information to monitor and adjust the electrical energy in the distribution system.
One reason to monitor line frequency is the increased interest and concern with the accurate measurement of harmonics on the utility's power system. Historical metering practices had only a minor concern with harmonics, but today interest is much higher due to an increase in customer loads which generate harmonics on a utility's system. These harmonics can cause VA loads on transformers to be higher than expected as well as cause a customer's bill to actually go down when harmonic power is actually being drawn from the utility's system. Frequency compensation is desired to obtain accurate measurements of amounts of harmonics on voltage or current signals.
In the past few years, electronic energy meters have moved more into the digital world with analog-to-digital converters (ADCs) and digital processing. More recently, digital electricity meters have started including additional instrumentation features which allow the user to read near instantaneous value readings such as phase angles from one voltage to another voltage, phase angles from a current to a voltage, per phase power factors, per phase voltages, per phase currents, per phase voltage harmonics, per phase current harmonics, per phase and system watts, per phase and system volt-amperes (VAs), per phase and system volt-amperes reactive (VARs), and total harmonic distortions for per phase voltages and currents. One problem that must be considered is the problem of frequency dependency, especially on values such as per phase voltage and current harmonics.
Digital meters tend to repetitively process samples at fixed time intervals, and although some quantities may be calculated one set of samples at a time, other quantities are desirably averaged over one or more line cycle periods. Since a fixed sampling rate implies a fixed number of samples per line cycle period, the results are generally compensated for variations in line frequency to avoid errant results. A typical means to adjust RMS voltages, RMS currents, and Volt-Ampere (VA) apparent energy is to detect zero crossings of a signal and average the results by the number of samples incurred over that flexible period. However, other more complicated calculations, such as harmonics, cannot be totally compensated for after the measurements and interim calculations are done.